Astronomers have repeatedly reported evidence that black holes lurk at the centers of galaxies. However, the latest finding all but settles the case, many scientists assert. Using a continent-wide array of radio telescopes, a U.S.-Japanese team last week reported "compelling evidence" that the center of a relatively nearby galaxy harbors a black hole as massive as 40 million suns.

The unusually high resolution of the network of 10 radio telescopes, collectively known as the Very Long Baseline Array (VLBA), enabled researchers to record structures deep within the spiral galaxy NGC 4258, which lies some 21 million light-years from Earth.

Spotting details 1,000 times smaller than those detected by visible-light telescopes, the team measured with unprecedented accuracy the swirling motion of a disk of gas and dust orbiting the galactic core. The inner part of the disk lies only about one-third of a light-year from the galaxy's center.

From this motion, clocked at 900 kilometers per second, the team infers that the core has a minimum density of 100 million suns per cubic light-year. That number exceeds the density of any other galactic center ever measured.

A cluster of ordinary stars with that density would have been torn apart long ago by collisions between individual stars, notes James M. Moran of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. His team says it has no choice but to conclude that the heart of NGC 4258 contains a supermassive black hole.

Moran and his colleagues reported their findings last week at a meeting of the American Astronomical Society in Tucson. They provide further details in the Jan. 12 Nature and the upcoming Feb. 20 Astrophysical Journal.

"The work...provides what may constitute the strongest case yet for a supermassive black hole in a galactic nucleus," writes Richard Barvainis of the Massachusetts Institute of Technology's Haystack Observatory in Westford. His commentary accompanies the Nature report.

"I was really blown away by their use of the [radio telescope] technology," says Patrick S. Osmer of Ohio State University in Columbus. "Maybe the public thinks that black holes have already been proved to exist, but this current study is much more definite. [The researchers] have probed much closer to the center of the galaxy than before."

If they indeed exist, black holes by their very nature can't be seen; astronomers must deduce their presence from their gravitational tug on surrounding stars, dust, or gas. Last year, another team made headlines when it used the Hubble Space Telescope to infer the existence of a massive black hole at the core of the elliptical galaxy M87 (SN: 6/4/94, p.356).

In the new study, Moran and his colleagues used the VLBA to study radio emissions produced by water molecules within a central galactic disk. The molecules act as masers, amplifying microwave radio emissions in much the same way a laser amplifies visible light and produces intense, pencil-thin beams of radiation. Because of the edge-on orientation of the disk, some of these beams shoot toward Earth, providing a clear view of the dynamics at the center of the galaxy. Unlike visible light, radio waves easily penetrate the dust present at the core of many galaxies.

"The work is enormously compelling," says Martin J. Rees of the University of Cambridge in England. "It's much better than all the public hype from the Hubble Space Telescope and M87."

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